DEFINITIONS.

The quantity of matter is the measure of the same, arising from its
density and bulk conjunctly.

THUS air of double density, in a double space, is quadruple in quantity; in a triple
space, sextuple in quantity. The same thing is to be understood of snow, and fine dust or
powders, that are condensed by compression or liquefaction; and of all bodies that are by
any caused whatever differently condensed. I have no regard in this place to a medium, if
any such there is, that freely pervades the interstices between the parts of bodies. It is
this quantity that I mean hereafter everywhere under the name of body or mass. And the
same is known by the weight of each body; for it is proportional to the weight, as I have
found by experiments on pendulums, very accurately made, which shall be shewn hereafter.

The vis insita, or innate force of matter, is a power of
resisting, by which every body, as much as in it lies, endeavours to persevere in its
present state, whether it be of rest, or of moving uniformly forward in a right line.

This force is ever proportional to the body whose force it is; and differs nothing from
the inactivity of the mass, but in our manner of conceiving it. A body, from the
inactivity of matter, is not without difficulty put out of its state of rest or motion.
Upon which account, this vis insita, may, by a most significant name, be called vis
inertiæ, or force of inactivity. But a body exerts this force only, when another
force, impressed upon it, endeavours to change its condition; and the exercise of this
force may be considered both as resistance and impulse; it is resistance, in so far as the
body, for maintaining its present state, withstands the force impressed; it is impulse, in
so far as the body, by not easily giving way to the impressed force of another, endeavours
to change the state of that other. Resistance is usually ascribed to bodies at rest, and
impulse to those in motion; but motion and rest, as commonly conceived, are only
relatively distinguished; nor are those bodies always truly at rest, which commonly are
taken to be so.

An impressed force is an action exerted upon a body, in order to
change its state, either of rest, or of moving uniformly forward in a right line.

This force consists in the action only; and remains no longer in the body when the
action is over. For a body maintains every new state it acquires, by its vis inertiæ
only. Impressed forces are of different origins asfrom percussion, from pressure,
from centripetal force.

A centripetal force is that by which bodies are drawn or impelled, or
any way tend, towards a point as a centre.

Of this sort is gravity, by which bodies tend to the centre of the earth; magnetism, by
which iron tends to the load-stone; and that force, whatever it is, by which the planets
are perpetually drawn aside from the rectilinear motions, which otherwise they would
pursue, and made to revolve in curvilinear orbits. A stone whirled about in a sling,
endeavours to recede from the hand that turns it; and by that endeavour, distends the
sling, and that with so much the greater force, asit is revolved with the greater
velocity, and as soon as ever it is let go, flies away. That force which opposes itself to
this endeavour, and by which the sling perpetually draws back the stone towards the hand,
and retains it in its orbit, because it is directed to the hand as the centre of the
orbit, I call the centripetal force. And the thing is to be understood of all bodies,
revolved in any orbits. They all endeavour to recede from the centres of their orbits; and
were it not for the opposition of a contrary force which restrains them to, and detains
them in their orbits, which I therefore call centripetal, would fly off in right lines,
with a uniform motion. A projectile, if it was not for the force of gravity, would not
deviate towards the earth, but would go off from it in a right line, and that with an
uniform motion, if the resistance of the air was taken away. It is by its gravity that it
is drawn aside perpetually from its rectilinear course, and made to deviate towards the
earth more or less, according to the force of its gravity, and the velocity of its motion.
The less its gravity is, for the quantity of its matter, orthe greater the
velocity with which it is projected, the less will it deviate from a rectilinear course,
and the farther it will go. If a leaden ball, projected from the top of a mountain by the
force of gunpowder with a given velocity, and in a direction parallel to the horizon, is
carried in a curve line to the distance of two miles before it falls to the ground; the
same, if the resistance of the air were taken away, with a double or decuple velocity,
would fly twice or ten times as far. And by increasing the velocity, we may at pleasure
increase the distance to which it might be projected, and diminish the curvature of the
line, which it might describe, till at last it should fall at the distance of 10, 30, or
90 degrees, or even might go quite round the whole earth before it falls; or lastly, so
that it might never fall to the earth, but go forward into the celestial spaces, and
proceed in its motion in infinitum. And after the same manner that a projectile, by
the force of gravity, may be made to revolve in an orbit, and go round the whole earth,
the moon also, either by the force of gravity, if it is endued with gravity, or by any
other force, that impels it towards the earth, may be perpetually drawn aside towards the
earth, out of the rectilinear way, which by its innate force it would pursue; and would be
made to revolve in the orbit which it now describes; nor could the moon without some such
force, be retained in its orbit. If this force was too small, it would not sufficiently
turn the moon out of a rectilinear course: if it was too great, it would turn it too much,
and draw down the moon from its orbit towards the earth. It is necessary, that the force
be of a just quantity, and it belongs to the mathematicians to find the force, that may
serve exactly to retain a body in a given orbit, with a given velocity; and vice versa,
to determine the curvilinear way, into which a body projected from a given place, with a
given velocity, may be made to deviate from its natural rectilinear way, by means of a
given force.

The quantity of any centripetal force may be considered as of three kinds; absolute,
accelerative, and motive.

The accelerative quantity of a centripetal force is the measure of
the same, proportional to the velocity which it generates in a given time.

Thus the force of the same load-stone is greater at a less distance, and less at a
greater: also the force of gravity is greater in valleys, less on tops of exceeding high
mountains; and yet less (as shall hereafter be shown), at greater distances from the body
of the earth; but at equal distances, it is the same everywhere; because (taking away, or
allowing for the resistance of the air), it equally accelerates all falling bodies,
whether heavy or light, great or small.

The motive quantity of a centripetal force, is the measure of the
same, proportional to the motion which it generates in a given time.

Thusthe weight is greater in a greater body, less in a less body; and, in the
same body, it is greater near to the earth, and less at remoter distances. This sort of
quantity is the centripetency, or propension of the whole body towards the centre, or, as
I may say, its weight; and it is always known by the quantity of an equal and contrary
force just sufficient to hinder, the descent of the body.

These quantities of forces, we may, for brevity's sake, call by the names of motive,
accelerative, and absolute forces; and, for distinction's sake, consider them, with
respect to the bodies that tend to the centre; to the places of those bodies; and to the
centre of force towards which they tend; that is to say, I refer the motive force to the
body as an endeavour and propensity of the whole towards a centre, arising from the
propensities of the several parts taken together; the accelerative force to the place of
the body, as a certain power or energy diffused from the centre to all places around to
move the bodies that are in them; and the absolute force to the centre, as endued with
some cause, without which those motive forces would not be propagated through the spaces
round about; whether that cause be some central body (such as is the load-stone, in the
centre of the magnetic force, or the earth in the centre of the gravitating force), or
anything else that does not yet appear. For I here design only to give a mathematical
notion of those forces, without considering their physical causes and seats.

Wherefore the accelerative force will stand in the same relation to the motive, as
celerity does to motion. For the quantity of motion arises from the celerity drawn into
the quantity of matter; and the motive force arises from the accelerative force drawn into
the same quantity of matter. For the sum of the actions of the accelerative force, upon
the several particles of the body, is the motive force of the whole. Hence it is, that
near the surface of the earth, where the accelerative gravity, or force productive of
gravity, in all bodies is the same, the motive gravity or the weight is as the body: but
if we should ascend to higher regions, where the accelerative gravity is less, the weight
would be equally diminished, and would always be as the product of the body, by the
accelerative gravity. So in those regions, where the accelerative gravity is diminished
into one half, the weight of a body two or three times less, will be four or six times
less.

I likewise call attractions and impulses, in the same sense, accelerative, and motive;
and use the words attraction, impulse or propensity of any sort towards a centre,
promiscuously, and indifferently, one for another; considering those forces not
physically, but mathematically: wherefore, the reader is not to imagine, that by those
words, I anywhere take upon me to define the kind, or the manner of any action, the causes
or the physical reason thereof, or that I attribute forces, in a true and physical sense,
to certain centres (which are only mathematical points); when at any time I happen to
speak of centres as attracting, or as endued with attractive powers.

Hitherto I have laid down the definitions of such words as are less known, and
explained the sense in which I would have them to be understood in the following
discourse. I do not define time, space, place and motion, as being well known to all. Only
I must observe, that the vulgar conceive those quantities under no other notions but from
the relation they bear to sensible objects. And thence arise certain prejudices, for the
removing of which, it will be convenient to distinguish them into absolute and relative,
true and apparent, mathematical and common.

I. Absolute, true, and mathematical time, of itself, and from its
own nature flows equably without regard to anything external, and by another name is
called duration: relative, apparent, and common time, is some sensible and external
(whether accurate or unequable) measure of duration by the means of motion, which is
commonly used instead of true time; such as an hour, a day, a month, a year.

II. Absolute space, in its own nature, without regard to anything
external, remains always similar and immovable. Relative space is some movable dimension
or measure of the absolute spaces; which our senses determine by its position to bodies;
and which is vulgarly taken for immovable space; such is the dimension of a
subterraneaneous, an æreal, or celestial space, determined by its position in respect of
the earth. Absolute and relative space, are the same in figure and magnitude; but they do
not remain always numerically the same. For if the earth, for instance, moves, a space of
our air, which relatively and in respect of the earth remains always the same, will at one
time be one part of the absolute space into which the air passes; at another time it will
be another part of the same, and so, absolutely understood, it will be perpetually
mutable.

III. Place is a part of space which a body takes up, and is
according to the space, either absolute or relative. I say, a part of space; not the
situation nor the external surface of the body. For the places of equal solids are always
equal; but their superfices, by reason of their dissimilar figures, are often unequal.
Positions properly have no quantity, nor are they so much the places themselves, as the
properties of places. The motion of the whole is the same thing with the sum of the
motions of the parts; that is, the translation of the whole, out of its place, is the same
thing with the sum of the translations of the parts out of their places; and therefore the
place of the whole is the same thing with the sum of the places of the parts, and for that
reason, it is internal, and in the whole body.

IV. Absolute motion is the translation of a body from one absolute
place into another; and relative motion, the translation from one relative place into
another. Thus in a ship under sail, the relative place of a body is that part of the ship
which the body possesses; or that part of its cavity which the body fills, and which
therefore moves together with the ship: and relative rest is the continuance of the body
in the same part of the ship, or of its cavity. But real, absolute rest, is the
continuance of the body in the same part of that immovable space, in which the ship
itself, its cavity, and all that it contains, is moved. Wherefore if the earth is really
at rest, the body, which relatively rests in the ship, will really and absolutely move
with the same velocity which the ship has on the earth. But if the earth also moves, the
true and absolute motion of the body will arise, partly from the true motion of the earth,
in immovable space; partly from the relative motion of the ship on the earth; and if the
body moves also relatively in the ship; its true motion will arise, partly from the true
motion of the earth, in immovable space, and partly from the relative motions as well of
the ship on the earth, as of the body in the ship; and from these relative motions will
arise the relative motion of the body on the earth. As if that part of the earth, where
the ship is, was truly moved toward the east, with a velocity of 10010 parts; while the
ship itself, with fresh gale, and full sails, is carried towards the west, with a velocity
expressed by 10 of those parts; but a sailor walks in the ship towards the east, with 1
part of the said velocity; then the sailor will be moved truly in immovable space towards
the east, with a velocity of 10001 parts, and relatively on the earth towards the west,
with a velocity of 9 of those parts.

Absolute time, in astronomy, is distinguished from relative, by the equation or
correlation of the vulgar time. For the natural days are truly unequal, though they are
commonly considered as equal and used for a measure of time; astronomers correct this
inequality for their more accurate deducing of the celestial motions. It may be, that
there is no such thing as an equable motion, whereby time may be accurately measured. All
motions may be accelerated and retarded, but the true, or equable, progress of absolute
time is liable to no change. The duration or perseverance of the existence of things
remains the same, whether the motions are swift or slow, or none at all: and therefore, it
ought to be distinguished from what are only sensible measures thereof; and out of which
we collect it, by means of the astronomical equation. The necessity of which equation, for
determining the times of aphænomenon, is evinced as well from the experiments of
the pendulum clock, as by eclipses of the satellites of Jupiter.

As the order of the parts of time is immutable, so also is the order of the parts of
space. Suppose those parts to be moved out of their places, and they will be moved (if the
expression may be allowed) out of themselves. For times and spaces are, as it were, the
places as well of themselves as of all other things. All things are placed in time as to
order of succession; and in space as to order of situation. It is from their essence or
nature that they are places; and that the primary places of things should be moveable, is
absurd. These are therefore the absolute places; and translations out of those places, are
the only absolute motions.

But because the parts of space cannot be seen, or distinguished from one another by our
senses, therefore in their stead we use sensible measures of them. For from the positions
and distances of things from any body considered as immovable, we define all places; and
then with respect to such places, we estimate all motions, considering bodies as
transferred from some of those places into others. And so, instead of absolute places and
motions, we use relative ones; and that without any inconvenience in common affairs; but
in philosophical disquisitions, we ought to abstract from our senses, and consider things
themselves, distinct from what are only sensible measures of them. For it may be that
there is no body really at rest, to which the places and motions of others may be
referred.

But we my distinguish rest and motion, absolute and relative, one from the other by
their properties, causes and effects. It is a property of rest, that bodies really at rest
do rest in respect to one another. And therefore as it is possible, that in the remote
regions of the fixed stars, or perhaps far beyond them, there may be some body absolutely
at rest; but impossible to know, from the position of bodies to one another in our regions
whether any of these do keep the same position to that remote body; it follows that
absolute rest cannot be determined from the position of bodies in our regions.

It is a property of motion, that the parts, which retain given positions to their
wholes, do partake of the motions of those wholes. For all the parts of revolving bodies
endeavour to recede from the axis of motion; and the impetus of bodies moving forward,
arises from the joint impetus of all the parts. Therefore, if surrounding bodies are
moved, those that are relatively at rest within them, will partake of their motion. Upon
which account, the true and absolute motion of a body cannot be determined by the
translation of it from those which only seem to rest; for the external bodies ought not
only to appear at rest, but to be really at rest. For otherwise, all included bodies,
beside their translation from near the surrounding ones, partake likewise of their true
motions; and though that translation were not made they would not be really at rest, but
only seem to be so. For the surrounding bodies stand in the like relation to the
surrounded as the exterior part of a whole does to the interior, or as the shell does to
the kernel; but, if the shell moves, the kernel will also move, as being part of the
whole, without any removal from near the shell.

A property, near akin to the preceding, is this, that if a place is moved, whatever is
placed therein moves along with it; and therefore a body, which is moved from a place in
motion, partakes also of the motion of its place. Upon which account, all motions, from
places in motion, are no other than parts of entire and absolute motions; and every entire
motion is composed of the motion of the body out of its first place, and the motion of
this place out of its place; and so on, until we come to some immovable place, as in the
before-mentioned example of the sailor. Wherefore, entire and absolute motions can be no
otherwise determined than by immovable places; and for that reason I did before refer
those absolute motions to immovable places, but relative ones to movable places. Now no
other places are immovable but those that, from infinity to infinity, do all retain the
same given position to one another; and upon this account must ever remain unmoved; and do
thereby constitute immovable space.

The causes by which true, and relative motions are distinguished, one from the other,
are the forces impressed upon bodies to generate motion. True motion is neither generated
nor altered, but by some force impressed upon the body moved; but relative motion may be
generated or altered without any force impressed upon the body. For it is sufficient only
to impress some force on other bodies with which the former is compared, that by their
giving way, that relation may be changed, in which the relative rest or motion of this
other body did consist. Again, true motion suffers always some change from any force
impressed upon, the moving body; but relative motion does not necessarily undergo any
change by such forces. For if the same forces are likewise impressed on those other
bodies, with which the comparison is made, that the relative position may be preserved,
then that condition will be preserved in which the relative motion consists. And therefore
any relative motion may be changed when the true motion remains unaltered, and the
relative may be preserved when the true suffers some change. Upon which accounts, true
motion does by no means consist in such relations.

The effects which distinguish absolute from relative motion are,
the forces of receding from the axis of circular motion. For there are no such forces in a
circular motion purely relative, but in a true and absolute circular motion, they are
greater or less, according to the quantity of the motion. If a vessel, hung by a long
cord, is so often turned about that the cord is strongly twisted, then filled with water,
and held at rest together with the water; after, by the sudden action of another force, it
is whirled about the contrary way, and while the cord is untwisting itself, the vessel
continues, for some time in this motion; the surface of the water will at first be plain,
as before the vessel began to move: but the vessel, by gradually communicating its motion
to the water, will make it begin sensibly to evolve, and recede by little and little from
the middle, and ascend to the sides of the vessel, forming itself into a concave figure
(as I have experienced), and the swifter the motion becomes, the higher will the water
rise, till at last, performing its revolutions in the same times with the vessel, it
becomes relatively at rest in it. This ascent of the water shows its endeavour to recede
from the axis of its motion; and the true and absolute circular motion of the water, which
is here directly contrary to the relative, discovers itself, and may be measured by this
endeavour. At first, when the relative motion of the water in the vessel was greatest, it
produced no endeavour to recede from the axis; the water showed no tendency to the
circumference, nor any ascent towards the sides of the vessel, but remained of a plain
surface, and therefore its true circular motion had not yet begun. But afterwards, when
the relative motion of the water had decreased, the ascent thereof towards the sides of
the vessel proved its endeavour to recede from the axis; and this endeavour showed the
real circular motion of the water perpetually increasing, till it had acquired its
greatest quantity, when the water rested relatively in the vessel. And therefore this
endeavour, does not depend upon any translation of the water in respect of the ambient
bodies, nor can true circular motion be defined by such translation. There is only one
real circular motion of any one revolving body, corresponding to only one power of
endeavouring to recede from its axis of motion, as its proper and adequate effect; but
relative motions, in one and the same body, are innumerable, according to the various
relations it bears to external bodies, and like other relations, are altogether destitute
of any real effect, any otherwise than they may partake of that one only true motion. And
therefore in their system who suppose that our heavens, revolving below the sphere of the
fixed stars, carry the planets along with them; the several parts of those heavens and the
planets, which are indeed relatively at rest in their heavens, do yet really move. For
they change their position one to another (which never happens to bodies truly at rest),
and being carried together with their heavens, partake of their motions, and as parts of
revolving wholes, endeavour to recede from the axis of their motions.

Wherefore relative quantities are not the quantities themselves, whose names they bear,
but those sensible measures of them (either accurate or inaccurate), which are commonly
used instead of the measured quantities themselves. And if the meaning of words is to be
determined by their use, then bythe namestime, space, place and motion,
their measures are properly to be understood; and the expression will be unusual, and
purely mathematical, if the measured quantities themselves are meant. Upon which account,
they do strain the sacred writings, who there interpret those words for the measured
quantities. Nor do those less defile the purity of mathematical and philosophical truths,
who confound real quantities themselves with their relations and vulgar measures.

It is indeed a matter of great difficulty to discover, and effectually to distinguish,
the true motion of particular bodies from the apparent; because the parts of that
immovable space, in which those motions are performed, do by no means come under the
observation of our senses. Yet the thing is not altogether desperate; for we have some
arguments to guide us, partly from the apparent motions, which are the differences of the
true motions; partly from the forces, which are the causes and effects of the true motion.
For instance, if two globes, kept at a given distance one from the other by means of a
cord that connects them, were revolved about their common centre of gravity, we might,
from the tension of the cord, discover the endeavour of the globes to recede from the axis
of their motion, and from thence we might compute the quantity of their circular motions.
And then if any equal forces should be impressed at once on the alternate faces of the
globes to augment or diminish their circular motions, from the increase or decrease of the
tension of the cord, we might infer the increment or decrement of their motions; and
thence would be found on what faces those forces ought to be impressed, that the motions
of the globes might be most augmented; that is, we might discover their hindermost faces,
or those which, in the circular motion, do follow. But the faces which follow being known
and consequently the opposite ones that precede, we should likewise know the determination
of their motions. And thus we might find both the quantity and the determination of this
circular motion, even in an immense vacuum, where there was nothing external orsensible
with which the globes could be compared. But now, if in that space some remote bodies were
placed the kept always a given position one to another, as the fixed stars do in our
regions, we could not indeed determine from the relative translation of the globes among
those bodies, whether the motion did belong to the globes or to the bodies. But if we
observed the cord, and found that its tension was that very tension which the motions f
the globes required, we might conclude the motion to be in the globes, and the bodies to
be at rest; and then, lastly, from the translation of the globes among the bodies, we
should find the determination of their motions. But how we are to collect the true motions
from their causes, effects, and apparent differences; and, vice versa, how from the
motions, either true or apparent, we may come to the knowledge of their causes and
effects, shall be explained more at large in the following tract. For to this end it was
that I composed it.